OvmfPkg: sync PcdVariableStoreSize with PcdFlashNvStorageVariableSize

OvmfPkg: sync PcdVariableStoreSize with PcdFlashNvStorageVariableSize

"MdeModulePkg/MdeModulePkg.dec" declares PcdVariableStoreSize like this:

> The size of volatile buffer. This buffer is used to store VOLATILE
> attribute variables.

There is no inherent reason why the size of the volatile variable store
should match the same of the non-volatile variable store. Indeed flash
variables in the 4MB build work fine without this equality.

However, OvmfPkg/EmuVariableFvbRuntimeDxe uses PcdVariableStoreSize to
initialize the non-volatile VARIABLE_STORE_HEADER too. (Presumably based
on the fact that ultimately that storage will not be permanent.) When
using EmuVariableFvbRuntimeDxe in the 4MB build, the mismatch between the
two mentioned PCDs (which is apparent through EmuVariableFvbRuntimeDxe's
VARIABLE_STORE_HEADER) triggers an assertion in the variable driver:

> ASSERT MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c(3772):
> mNvVariableCache->Size == VariableStoreLength

Bringing PcdVariableStoreSize in sync with PcdFlashNvStorageVariableSize
fixes this. It also happens to ensure a volatile store size in the 4MB
build that equals the non-volatile store size, which likely doesn't hurt
for symmetry.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Fixes: b24fca05751f8222acf264853709012e0ab7bf49
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>